This invention relates to high temperature electrochemical converters, and specifically to high performance power systems employing such devices and methods.
Conventional, electrochemical converters perform fuel-to-electricity conversions in a fuel cell (electric generator) mode or electricity-to-fuel conversions in an electrolyzer (fuel synthesizer) mode. The converters are capable of high efficiencies, depending only on the relation between the free energy and enthalpy of the electrochemical reaction, and are not limited by Carnot-cycle considerations.
The key components in an electrochemical energy converter are a series of electrolyte units having electrodes disposed over its surfaces, and a similar series of interconnectors disposed between the electrolyte units to provide serial electrical connections. Each electrolyte unit is an ionic conductor having low ionic resistance thereby allowing the transport of an ionic species from one electrode-electrolyte interface to the opposite electrode-electrolyte interface under the operating conditions of the converter. Various electrolytes can be used in such converters. For example, zirconia stabilized with such compounds as magnesia, calcia or yttria can satisfy these requirements when operating at an elevated temperature (typically around 1000.degree. C.). The electrolyte material utilizes oxygen ions to carry electrical current. The electrolyte should not be conductive to electrons which can cause a short-circuit of the converter. On the other hand, the interconnector must be a good electronic conductor. The interaction of the reacting gas, electrode and electrolyte occurs at the electrode-electrolyte interface, which requires that the electrodes be sufficiently porous to admit the reacting gas species and to permit exit of product species.
The "traditional" method for fuel cell thermal management is to force a cooling medium, either a liquid or gaseous coolant stream, through the fuel cell assembly. Cooling water is often employed for ambient temperature devices, and air can be employed for higher temperature fuel cells. In some instances, the same air which serves as the fuel cell's oxidant is used as a cooling medium as well. The cooling medium passes through the fuel cell and carries off the thermal energy by its sensible heat capacity. The volume flow of coolant required for this method is inversely related to the limited temperature operating range of the electrochemical operation of the electrolyte, or in the case of fuel cells with ceramic components, by constraints associated with thermal stress.
The foregoing heat capacity limitations on the amount of temperature rise of the cooling medium result in coolant flow rates through the fuel cell much higher than those required by the electrochemical reaction alone. Since these relatively large flow quantities must be preheated to a temperature near the operating temperature of the fuel cell and circulated therethrough, a dedicated reactant thermal management subsystem is required. Such thermal management subsystems normally include equipment for regenerative heating, pumping and processing of the excessive coolant flow. These additional components add substantially to the overall cost of the system.
For illustration purposes, consider a regenerative heat exchanger of a type suitable for preheating the fuel cell reactants and operating with a 100.degree. C. temperature difference, and a typical heat transfer rate of 500 Btu/hr-ft.sup.2 (0.13 W/cm.sup.2). Further assuming a 50% cell efficiency with no excess coolant flow, and operating at an ambient pressure, the heat processing or heat transfer surface area of the regenerator would be of the same order of magnitude as the surface area of the fuel cell electrolyte. Considering an excess coolant flow requirement of 10 times the level required for the fuel cell reactant flow, which is normally required to sufficiently cool the fuel cell in conventional approaches, the heat exchanger surface area would be 10 times larger than the active fuel cell surface area. The large size of this heat exchanger makes it difficult to integrate the heat exchanger with electrochemical converters to form a compact and efficient thermal management system or to integrate the electrochemical converter with a dedicated power system.
Conventional high performance power systems exist and are known, and include steam and gas turbines. A conventional gas turbine power system includes a compressor, a combustor, and a gas turbine, typically connected in-line, e.g., connected along the same axis. In a conventional gas turbine, air enters the compressor and exits at a desirable elevated pressure. This high-pressure air stream enters the combustor, where it reacts with fuel, and is heated to a selected elevated temperature. This heated gas stream then enters the gas turbine and expands adiabatically, thereby performing work. One drawback of gas turbines of this general type is that the turbine typically operates at relatively low system efficiencies, usually around 25%.
One prior art method employed to overcome this problem is to directly channel high-temperature exhaust exiting the combustor into a recuperator for recovering heat. This recovered heat is typically used to further heat the air stream prior to the stream entering the combustor. A drawback of this solution is that the recuperator is relatively expensive and thus adds to the overall cost of the power system.
Another prior art method employed is to operate the system at a relatively high pressure and a relatively high temperature to thereby increase system efficiency. However, the actual increase in system efficiency has been nominal, while the system is subjected to the costs associated with maintaining this high pressure and temperature environment.
Thus, there exists a need in the art for better thermal management and integration approaches, especially for use in high performance power systems. In particular, an improved power system that is capable of integrating and employing the desirable properties of electrochemical converters would represent a major improvement in the industry. More particularly, an improved electrochemical converter and gas turbine system that reduces the costs associated with providing dedicated thermal processing control systems would also represent a major improvement in the art. Furthermore, a power system that utilizes an electrochemical converter and that has increased system efficiency would also be desirable.